Biodegradable organic wastes are produced in large quantities annually by industrial, agricultural and domestic activities. In the treatment of municipal sewage, for instance, at least three such waste streams are produced, namely grits, screenings, and sewage sludge. All biodegradable materials are capable of undergoing putrefaction, however, the process only occurs in the presence of at least 10% water. Normally, significant putrefaction can be observed when such a waste contains over 20% water. It is known that in the process of putrefaction many volatile components such as hydrogen sulphide, mercaptans, volatile fatty acids and ammonia are generated. Such compounds are responsible for the offensive odour that is characteristic of rotting matters. In industrialised countries such organic wastes are disposed off in incinerators, landfills or recycled to agriculture as fertiliser or soil conditioner. However, incineration and land filling are regarded as needless destruction of valuable resources and environmentally harmful. Thus, in Europe for instance, recycling is encouraged while both incineration and land filling are being restricted.
There are regulations governing the recycling of any waste products to agriculture. They vary from country to country, but an important underlying principle of recycling is that it should bring an agricultural benefit and have regards for environment health and odour nuisance. Agricultural benefit will be achieved when an application of a waste to land improves soil conditions for crop growth whilst ensuring the protection of the environment. If the wastes contain pathogens, as is the case with sewage sludge, health risks may exist for farm workers and animals. Suitable treatment processes must therefore be applied to the wastes before recycling. Such treatments must be capable of significantly reducing the E. coli content of the waste and preventing odour generation by imparting stability to the material.
Known treatments for organic wastes include biological, physical and chemical processes. Biological processes such as anaerobic digestion or composting impart stability to the waste by reducing its organic content. Thermal drying is a physical treatment that does not alter the chemical make up of the waste and stabilisation can only be achieved by reducing and maintaining the water content of the waste to a level below 10%. Lime is the most common chemical treatment for sewage sludge. In this case, the organic content of the sludge remains unchanged. The lime typically raises the pH of sludge to over pH 12 whereby pathogens such as E. coli are deactivated.
The mentioned treatment processes all have disadvantages and drawbacks. Digestion and composting, for instance, are very slow processes. Typically, they take several weeks to complete and require very costly processing plants. On the other hand, drying is very energy intensive and sludge drying, in particular, is often a hazardous process because of the risk of explosion with the ever presence of dust. Lime treatment is only favourable for applications on acid soils. On exposure to air, particularly during a long stockpiling period, reaction between the lime and carbon dioxide rapidly reduces the pH of sludge to a level where biological activities can resume and the resulting odour release may cause a nuisance.
There have also been attempts to convert organic wastes to fertilisers. Such methods combine chemical treatment at elevated temperature to kill pathogen and odour causing bacteria and simultaneously achieve nutrient enhancement. EP 0143392 A1 (ECODECO spa) describes a salting process for sludge slurry having dry solid content of 0.1% to 20% by weights. The process uses high salt concentration, for example, saturated ammonium sulphate with acidic pH. WO 93/14046 A1 (KAMBI AS) describes a very high temperature acid treatment to dissolve the solids followed by neutralization of the resulting hydrolysate with ammonia. In yet another application (US 2002/0088749 A1) Highsmith describe a treatment method involving ammonium and phosphate salts with acids. The Highsmith method applies to anaerobically digested sludge rather than raw sludge and it does not address the odour issue in any satisfactory manner. While the increased nutrient value of such organic fertilisers make them valuable, the high cost of their manufacture has not facilitated their commercialisation.